10/90 tube graduation error.

[dgs]

I've realised for a while that some or all of the 10/90 dropout tubes we use arn't too accurate with their graduations. Of course the nearer we get to 100% conversion (no dropout) then the less significant this error is, however although I've known the 10ml mark is too low on most of the ones I've checked, I didn't realise how far out is was.

This error is especially significant when doing a test on oil after glyc pre-treating, to see how much conversion has been achieved. I usually have 10mls of dropout after the glyc wash (totally forgetting about the error in the tube - stupid me!)

This only 'struck' me yesterday when testing oil that I had washed with a lot of glycerol, ex a 20% methanol process. this only showed 8mls dropout, so I syringed 10mls into a dry tube (same tube)  with no methanol, this showed 13, yes 13mls!

I'll have to watch this in future. I also wonder how accurate 10ml syringes are.

I always used to pipette the sample but when using oil this was very inaccurate as the viscosity would not let the full sample flow out.

[Julian]

You could check a syringe mathematically.  I assume the volume of the nozzle is accounted for when the graduations are added.

[Jamesrl]

You could check a syringe mathematically.  I assume the volume of the nozzle is accounted for when the graduations are added.

If you divide the volume wanted by the cross sectional area you'll get the lenght of that volume in the syringe.

[Tony]

I assumed the air bubble accounted for the volume of the nozzle up to the rubber seal, and it's the movement of the rubber seal between 0 and 10ml, say, that gives the syringe volume as Jim says above.  IE, full, with no air bubble, would be greater than the marked volume.

[Julian]

Good point!

[dgs]

I assumed the air bubble accounted for the volume of the nozzle up to the rubber seal, and it's the movement of the rubber seal between 0 and 10ml, say, that gives the syringe volume as Jim says above.  IE, full, with no air bubble, would be greater than the marked volume.

What air bubble is this? The plunger on the 2 part syringes has a point and this has to be exactly level with any given mark on the side to deliver exactly that amount. If I ever have an air bubble I get rid and start again.

[dgs]

I did some comparitive volume tests today:

I used IPA as the liquid for all the following,

Test 1 10mls syringed into the 10/90 tube, gave 13mls, poured into a 25ml graduated measuring cylinder, gave 12mls.

Test 2, 10 mls syringed into a beaker, then 'sucked' up with a 10ml pipette, it showed about 11mls.

Test 3, 10mls in pipette, ran into dropout tube and showed 11mls, poured into 25ml cylinder and showed 10mls.

As the pipette and measuring cylinder seem to match it looks more likely that it is the syringe that is out by about 2mls (so the 10ml mark is more like 12mls)

[Tony]

If you remove the air bubble, are you leaving a small amount of liquid in the syringe?  Or are you pulling air in after the first squirt in order to expel the entire contents?

Have you got some jeweler's scales, could measure distilled water mass to see if that agrees with the cylinders?

[dgs]

If you remove the air bubble, are you leaving a small amount of liquid in the syringe?  Or are you pulling air in after the first squirt in order to expel the entire contents?

Have you got some jeweler's scales, could measure distilled water mass to see if that agrees with the cylinders?

Good idea Tony, I'll try it when I get a few mins.

[RichardP]

I checked my 10/90 tubes this afternoon, as I have 3, all off which are from different manufacturers.



Using 10ml IPA with the syringe shown, the left hand and centre tubes both gave spot on 10ml, the right hand tube showed about 12ml.
All 3 showed the correct amount up to 6ml which is where the right hand one went wayward.

I checked with and without the air bubble and they showed negligible difference.

[dgs]

Thanks for that Richard. As said the error (if any) on low volumes of dropout is insignificant, the worry would be the dropout on glyc washed oil. Using full chemical amounts on a sample that showed 10mls dropout could lead to an overdose when the actual dropout could have been nearer the 8mls.

[RichardP]

I've just done a double glyc wash on a batch - two as there's some about 20l of stinking fat in it I need to get rid of in with the oil, the second one has had 240g of potassium in meth added. When I've dropped the glyc I'll do a 10/90 in each tube and see what the result is.

[RichardP]

So after glyc wash, the 10/90 using all 3 showed slightly different amounts than expected, albeit by 0,5ml.

Left tube - shows 9ml
Centre tube - 9.5ml
Right tube - 11ml

0.5ml in 10/90 sample won't really make much difference on a 2 stage reaction, as the more critical measurement is the stage 1 dropout measurement. All 3 tube looked ok at the smaller volumes.

[dgs]

Thanks Richard. Your last statement is correct, but if you used the right hand tube and assumed there had been no reaction and went ahead using full amounts of catalyst and methanol then this could result in an overdose, producing more soap.

This is exactly what has happened to me recently, after S1 using eq to 7.5gms K and only 12.5% methanol I have had no dropout, so I don't know if the reaction has just been on the cusp or if it has been well overdosed with chemicals. I like the dropout to ideally be about 0.5mls after S1 so I know exactly where I am at for S2

If there was a small reaction after the glyc pre-treatment then it is logical to assume the resultant oil has no ffa's present.

I too have three d/o tubes, I'll have to do some comparative tests today and report back.

[neisel]

Just checked my cheapo Chinese ebay bought glass syringe & discovered 10mls in China is 13mls in the UK. That would explain why I have been still getting a 100% pass while cutting down my final stage dosing by about 20% of what it 'should' be after consistently making too much soap when doing it by the numbers.

I know the flask is accurate 'cos it's one of these http://www.biopowered.co.uk/forum/index.php/topic,1504.msg17642.html#msg17642 but it just didn't occur to me that the syringe could be so badly off.

Duh. Live & learn.

Thanks for this thread.